Validation of the Operational Meris Fapar Products

This paper discusses the validation of the operational Medium Resolution Imaging Spectrometer (MERIS) land product which corresponds to the biophysical variable of the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). The FAPAR value acts as an indicator of the presence and state of the vegetation and it is currently estimated from MERIS data at both reduced and full resolution using a physically-based approach. The quality of the MERIS FAPAR products, derived from the MERIS Global Vegetation Index (MGVI) algorithm, capitalizes on the availability of MERIS data since June 2002. The validation protocol to assess the accuracy of FAPAR product is done 1) by analyzing the estimates of theoretical uncertainties (versus the algorithm formulae and instrument calibration performance), 2) by assessing the performance for detecting expected event using long time-series of FAPAR data over a well-known land surfaces and 3) by comparing the FAPAR MERIS values to similar products generated by other independent sensors like the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the MODerate Resolution Imaging Spectro-radiometer (MODIS) and against ground-estimates of FAPAR which have been performed over various types of land.